In a DSL deployment, stationary and impulsive noises are generated within the home environment, which impact the reliability of the WAN interface of a residential device delivering network services such as IPTV. Sources of such disturbances include house appliances such as vacuum cleaners, lamps, or equipment such as pool pumps, washing machines, etc. Apart from Differential Mode (DM) self-FEXT signals that are expected to result primarily from a DM to DM coupling, in a house environment DSL noise sources are assumed to originate predominantly from a capacitive coupling due to the proximity in the house of the twisted pair and the power supply mains on which are generated most of the domestic noise sources.
Some noise sources may be radiating externally directly into the twisted pair, which acts like an antenna to incoming waves. Such is the case for RFI disturbers that develop a common mode (CM) signal onto the drop and which get converted into a DM signal, without being necessarily present on the power mains. But it is expected that most of the domestic noise sources find their way to the DSL line due to capacitive coupling between the mains network and the twisted pair, rather than through radiation.
Relatedly, in traditional electromagnetic compatibility (EMC) compliance testing of DSL modems illustrated in FIGS. 1 and 2, injection of electrical fast transients (EFTs) into the equipment under test (EUT) 102 is performed in order to evaluate the immunity of modems against interferences that are representative of field conditions. Bursts of EFTs are typically caused by operation of electro-mechanical switches, motors and distribution switch-gears connected to the power distribution network. A typical burst consists of a large number of recurring impulses at high frequency for a short time period. Since the EFTs are inherently travelling on the power distribution network within a house, these transients can make their way to the DSL port with which they interfere through at least two possible paths: first through a capacitive or inductive coupling of the power supply lines in the house with the DSL cable itself, with which they come in close proximity; and secondly, through leakage of the EFT signals through the power supply leads to which the DSL modem is connected in order to receive its power. As a result of the multiplicity of possible coupling paths, immunity tests against EFTs are traditionally performed on the Telecom port (TP) and/or on the Power supply port. FIG. 1 illustrates the direct coupling of EFT signals into the DSL line itself by use of a coupling clamp 104. FIG. 2 illustrates the coupling of EFT signals through the power supply port 106 of the DSL modem.
The principle by which the EFT signal impacts the DM DSL signal is illustrated as follows. FIG. 3 illustrates the EFT signal conversion through the loop imbalance following a capacitive coupling of the EFT signal from the in-house mains network 302 and the DSL twisted pair 304. As the EFT signal travels on the power mains in-house network, a voltage VEFT is developing on the hot/neutral pair of the in-house network with respect to a reference ground. At one specific or more coupling points within the house due to the proximity of the in-house power mains network 302 and the DSL twisted pair 304, this VEFT signal couples into the DSL line and projects itself as a CM signal on the Tip and Ring (T & R) pair 308 of the DSL twisted pair, as a voltage VEFT-CM. The CM signal on the twisted pair is then converted to a DM voltage VEFT-DM due to the imbalance of the twisted pair with respect to ground. This VEFT-DM signal superimposes itself onto the useful DSL signal and perturbs it. This scenario is captured in the test procedure shown in FIG. 1.
FIG. 4 illustrates the principle by which the EFT signal travelling onto the in-house mains network 404 can leak through the power supply unit 402 of the DSL modem, and converts itself into a DM signal at the T & R of the DSL modem port. As represented, the voltage VEFT is developing on the hot/neutral pair of the in-house network 404 with respect to a reference ground. It is present on the power supply leads 406 that provide power to the DSL modem. Even if the power supply unit provides a high level of isolation for this unwanted signal, a certain voltage VEFT-CM can make its way through leakage to the DSL front end, which is electrically floating with respect to ground, thereby inducing a CM signal present on the T & R pair 408 of the DSL twisted pair at the modem. This CM signal VEFT-CM on the twisted pair is then converted to a DM voltage VEFT-DM due to the imbalance of the twisted pair with respect to ground, as seen at the point of injection. This VEFT-DM signal superimposes itself onto the useful DSL signal and perturbs it. This scenario is captured in the test procedure of FIG. 2.
In actual field scenarios, however, the injection of the EFT signals takes place simultaneously through capacitive coupling and power supply leakage, since the EFT signals are expected to impact both interfaces of the modem simultaneously. This situation just illustrates the fact that any modem (i.e. DSL link) may be susceptible to environmental interference on any of its physical interfaces (e.g. TP port, Power Supply port, Ethernet port, Serial port, etc.) In this event, whenever two coupling paths exist together between the power mains and the DSL loop, either through the capacitive coupling of the loop (FIG. 3) or through the leakage of the power supply (FIG. 4), the resulting DM noise on the DSL pair is actually a superposition of two separate noises which couple to the CM mode on the twisted pair, and then to DM through different transfer functions. FIGS. 3 and 4 illustrate that the CM to DM conversion of the signals that take place on the loop will be determined by two different mode conversion transfer functions, which will be different in the two cases due to the exact point of injection of the CM signal resulting from the coupling of the power mains noise. At these two points, which will be physically two distinct points on the cable (e.g. one point somewhere far from the modem (FIG. 3) and one point close to the modem (FIG. 4)), the imbalance of the cable that drives the conversion of the CM noise to DM as perceived by the modem is likely to be different.
Accordingly, as illustrated in FIG. 5, there will be in effect two coupling paths of interest: a signal conversion path 502 through capacitive/inductive coupling of the power mains into CM of T & R, which then gets converted to DM at a point of imbalance of the TP; and noise signal conversion 504 from the power mains through the power supply block into T & R, that may get converted from CM to DM locally due to T & R imbalance.
Those two paths of interest superimpose. Provided that the noise source signals, which couple into the two points of imbalance, are identical, the resultant noise will appear to have coupled through a single aggregate conversion path, due to the principle of superposition.
In practice, the coupling path through the power supply unit is under control of the board designer. It should be minimized and reduced to a level that is well below the coupling path that may exist between the power mains network and the DSL line along the in-house network. In this discussion, the manifestation of the noise leakage through the power supply unit serves the purpose of illustrating the point developed hereafter that a connection path exists to the power mains noise through the power supply. The present inventors recognize that if controlled, this connection can be put to use efficiently for noise source characterization and mitigation into the DSL line.
The situation of coupling of noise through the power supply is not limited to EFT noise sources; it was also observed by the present inventors in a controlled lab environment with an HP-AV disturber connected to the same power line as the modem. The leaked HP-AV signal created a measurable CM signal at the T & R, even though the T & R port of the modem was not connected to the cable. Whenever the modem was connected to the cable, the CM signal then got converted into a DM signal on the T & R through the loop imbalance. This situation is an illustration that in practice, noise sources that are effectively present on the power supply mains may find their way into the DSL port through power supply leakage.
In any event, in view of the foregoing, it would be desirable to be able to characterize the CM noise generated by the power mains that couples into the DM DSL signal at its source.